Blackboard instrument



Jan. 4, 1938. c F. GOLDEN BLACKBOARD INSTRUMENT Original Filed Nov. 20, 1935 lmiiifliL BE: IT- w 3pnentor attorneys Patented Jan. 4, 1938 UNITED STATES PATENT OFFICE Application November 20, 1935, Serial No. 50,769 Renewed June 4, 1937 1 Claim.

This invention relates to an instrument for use in drawing on blackboards and the like.

An object is to provide a T-square having a means whereby it can be easily and accurately centered on a predetermined point about or in line with which one or more lines can be inscribed by the use of the applied instrument.

A further object is to provide an instrument which will be held firmly to the surface on which it is used so as to guard against accidental displacement after it has been set.

With the foregoing and other objects in view which will appear as the description proceeds the invention resides in certain novel details of construction and combinations of parts which will be hereinafter more fully described and pointed out in the claim, it being understood that changes may be made in the construction of the device without departing from the spirit of the invention as claimed.

In the accompanying drawing the preferred form of the invention has been shown.

In said drawing:

Figure 1 is a plan view of the instrument.

Figure 2 is a section on line 2--2, Figure 1.

Figure 3 is a section on line 3-3, Figure 1.

Figure 4 is a detail of a modification.

Referring to the figures by characters of reference, l designates the semi-circular head of a T-square from the straight side 2 of which is extended the arm 3 having a straight-edge l perpendicular to the edge 2. The head and arm are formed in a singlepiece of metal and the arcuate edge 5 of the head is preferably bent down and beveled as shown.- The edge 2 and the edges of the arm 3 have depending longitudinal flanges as shown at 6 to stiffen the structure. Buttons I, preferably of rubber, are fastened to the head I and to the free end portion of arm 3 to provide means for contacting with the surface of the blackboard or other structure to which the instrument is applied.

A cap 3 is formed on or secured to the instrument at the juncture of the head and arm and provides a housing for a vacuum cup 9 of soft rubber or its equivalent. This cup has a tubular stem ll guided slidably in the cap 3 and having its longitudinal axis in line with the edge 2. A screw cap ll engages the outer or upper end of the stem and has a washer l2 for sealing it. A spring 13 bears at one end against the cap and at its other end against'the head ll of a pin l5 slidable in the stem. A pointed tip l6, which can be of rubber, is carried by the end of the pin and is projected normally away from the cap 3 at the center thereof. 1

A slide I1 is mounted on and embraces the arm 3 and a plate I8 is fitted under the arm and between the flanges 6. This plate has a threaded stem l9 extending therefrom through a slot 20 in arm 3 and through slide l1 and is engaged by a clamping nut 2| which can be utilized for binding the parts together after their adjustment along the arm. A sleeve 22 is supported by the slide beyond the edge 4 and serves to hold a crayon with its end substantially in line with the center of the stem l0.

In practice the projecting tip IS on pin I5 is applied to the point on the blackboard with reference to which lines are to be drawn. Stem I0 is then pressed toward the blackboard so as to compress the cup 9 which will grip the surface by suction. The buttons I will bear against the surface at widely separated points. The crayon is then placed in sleeve 22 and can be moved along the arm 3 to draw a straight line radiating from the point to which the tip I6 was applied or it can be adjusted to any distance from the said point and the entire instrument then moved in a circle about the axis of stem 10, thereby to inscribe an are or circle.

To remove the instrument the cap I l is slightly unscrewed to break the seal between the washer l2 and the end of the stem 10, thereby destroying the vacuum in the cup 9 and releasing the entire device.

In Figure 4 there has been illustrated a. portion of a modified structure showing how the head 23 can be made separately from its arm 23, the two being connected by rivets or the like insertible into holes 24 in the parts.-

The head and arm can be suitably graduated as at 25 to permit use of the device as a protractor.

Obviously the device can be used for describing circles; for drawing parallel lines when a traverse is being set off and parallel lines to such angles as may be desired; and for quick segmentation of circles. In other words an eflicient combination instrument for use on blackboards by engineers, has been evolved.

What is claimed is:

An instrument of the class described including a T-square, a tubular stem slidable therethrough, a vacuum cup on the stem, a centering pin normally projected from the cup, said stem being shlftable longitudinally to collapse the cup against a flat surface to create a partial vacuum therein and to retract the pin into the stem and sealing means on the stem shlftable to destroy the vacuum and release the cup.

CHARLES FREMONT GOLDEN. 

